Most passenger car and commercial vehicles have fluid systems such as a servo-assisted or power operated braking system. These systems include devices such as master cylinders, vacuum pumps, valves and compressors, some of which are driven by an internal combustion engine of the vehicle. In many cases, it is necessary to mount such devices directly on the engine, chassis or body of the vehicle, and this often results in part of the device being overspecified for the task which it must undertake. For example, valves of vacuum pumps may have a housing of cast iron simply because the body of the pump requires the strength and stability of cast iron, and because of additional assembly costs it is not convenient or practicable to make a valve housing of a more suitable and economical material, such as a thermoplastic.
Engine or chassis mounted devices have a moderately severe operating environment. They are exposed to vibration from the engine, and in the case of engine mounted devices, they are exposed to relatively high temperatures. Heat can be transferred from the engine to the engine mounted device either directly from the mounting flange, or indirectly hot engine oil.
For reasons of manufacturing ease and precision many engine and chassis mounted components for fluid systems are nowadays assembled but particular attention must be paid to sealing to prevent air and oil leaks. Usually elastomeric seals must be provided between component parts, but such assembled joints are not ideal and can be particularly troublesome in a high vibration environment; furthermore assembled joints require a larger number of components which inevitably increases manufacturing, stocking and assembly costs.
The use of plastics materials for engine and chassis mounted components has been contemplated in the past, but has shown no significant advantage over components made of for example cast iron or aluminium. Fasteners and flexible seals must be provided to hold together component parts of devices of fluid systems. Furthermore in structural applications plastics materials must have a large percentage of e.g. glass filler in order to exhibit the necessary resistance to creep.
This is especially true in high temperature/high stress applications. Plastics have been used for components having relatively low stress loadings, such as fluid reservoirs of brake master cylinders, and are sometimes welded. Such plastic parts are usually assembled in a conventional manner with fasteners and seals. However parts having a high filled content (such as those suitable for direct attachment to engine or chassis) are extremely difficult to weld since the plastic content is low, and thus fasteners and seals must again be provided, which necessarily increases manufacturing and assembly costs.
Thus, plastics have been used only for very specific applications where the disadvantages do not outweigh the advantages of, for example, reducing the weight of the finished article or improving the aesthetic appearance thereof, and have seldom been used for structural components.
Notwithstanding these Problems, the applicants have invented a form of construction for a device of a fluid system which is suitable for automotive applications and is best able to take advantage of the most appropriate materials whilst using thermoplastic for components which are not heavily stressed, such as fluid reservoirs.